The present invention relates to digital processing circuits for signals, particularly television signals, and more specifically a digital circuit for the alignment of the useful direct current component of an analog signal in such processing circuits.
In a signal processing system, the useful d.c. component of the process signal may become lost. It is indispensable to restore it before using the signal, in order to restore the information contained in said signal in an appropriate manner.
Conventionally, in digital processing circuits for initially analog signals, the d.c. component of the signal is restored or the alignment of a level of this signal (e.g. the block level for a television signal) with a reference level, is carried out by processing an analog signal, prior to digital conversion by an analog circuit. This circuit comprises a switch gate determining the intervals during which the reference level is fixed and a capacitance acting as a memory, which also has a filtering effect for the noise which may be present on the level taken as a basis for the alignment.
When this level alignment takes place in this way on the analog signal, it is necessary for the analog-digital converter which then processes the signal to convert it into a digital signal to be very stable. Otherwise, the drifts or variations introduced cannot be corrected and, even if stabilization has taken place on the analog signal, there still remains drifts in the output signal.
Moreover, such an analog alignment circuit involves a compromise between the establishment speed and the stability. Thus, a short establishment time requires a limited capacity, whereas a good stability requires a relatively large capacity. Moreover, in view of the leakage current of the circuit, the correction must take place at an adequate frequency.